Prior to the recent application of stable isotope based GC/MS methodology, little was known about in vivo essential fatty acid metabolism in animals or humans. Essential fatty acid metabolism was studies in human adults, both male and female, and those who smoked as well as non-smokers. This was a stable isotope study of in vivo metabolism of deuterated-LA and deuterated-LNA conversion after a single oral dose of these precursors. Our results indicated that female smokers had a two-fold increase in the percent of plasma dose and a higher fractional conversion rate for 22:5n-3 conversion to 22:6n-3 compared with non-smokers. Male smokers had elevated total plasma n-3 fatty acids, a more rapid turn over of D5-18:3n-3, a disappaerance rate of D5-20:5n-3 that was both delayed and slower, and a greater percentage of D5-20:5n-3 was directed into 22:5n-3 relative to non-smokers. Generally, smoking increased the bioavailablity of n-3 fatty acids from plasma, accelerated fractional conversion rates, and increased the percent formation for some long chain n-3 fatty acids. In rats, it was observed that addition of preformed DHA to the diet leads to a decreased accumulation of label from 18-C precursors into DHA and DPAn6 in several organs even though there was a significant increase in tissue DHA. Female rats accumulated more DHA and DPAn6 but less AA than males when fed a controlled diet containing 3 wt% alpha-linolenic acid. An n-3 fatty acid deficient diet led to a marked decline in labeling of liver 22:4n6 and 22:5n6 from the 18:2n6 precursor. A closely related research project concerns the origins of nervous system and other organ DHA. Possible sources are from dietary preformed DHA, from metabolism of the precursor, LNA, or from body stores of DHA. A novel technique has been developed that allows for the quantitative assessment of the amount of DHA accreted from LNA metabolism under various dietary conditions. For this study, it is necessary to control the diet from near birth up to a period where significant brain development has occurred. This has been accomplished thru the use of newly developed artifiicial rearing techniques using an artificial rat milk that was nearly devoid of n-3 fatty acids. The n-3 fatty acids are then added as deuterated-LNA and containing varying levels of DHA. In one major experiment, rat pups were fed diets with 0 or 2% DHA between days 8-29 of life. During this period, it could be calculated that 40% of the newly formed brain DHA in the animals fed D5-LNA as their only source of n-3 fatty acids were derived from preformed DHA and not from LNA metabolism. This was surprising as there was no DHA in the diet;thus, all preformed DHA deposited in the brain must have been derived from other organs via the blood stream. When DHA was added to the diet, there was a pronounced decrease in the rate of LNA metabolism to DHA, possibly due to a form of end-product inhibition, and 88% of brain DHA was derived from the preformed dietary DHA. The biochemical mechanisms underlying these metabolic effects of dietary DHA are being investigated. A decline in labeled DHA was also observed in liver, heart, muscle, kidney and testes but no such changes were observed in adipose tissues. There was also a higher level of brain DHA in the rats given preformed DHA indicating that metabolism could not provide an adequate source of brain DHA. Another finding of consequence for infants fed formulas without DHA was that several organs including the heart, lungs, kidney and spleen had a net loss of DHA content during a period of intense body growth when no preformed DHA was present in the diet. An attempt was made to determine what the underlying mechanisms for DHA transport into brain and other organs. Lipoproteins were purified and labeled with radiotracers and modified with a tracer levels of phospholipids acylated with DHA, AA or oleic acid (OA). The modified lipoproteins were intravenously injected in mice. The plasma and tissue distribution of the radiotracers were investigated as a function of time and the lipoproteins composition. We found that higher proportion of DHA in LDL results in an enhanced uptake of these lipoproteins by brain and heart. A similar enrichment of LDL in AA or OA did not result in any changes compared to control unaltered LDL. Tissue uptake of HDL did not depend on its fatty acid composition. We next compared the distribution in plasma pools and tissue uptake of 14C-DHA and 3H-(OA) intravenously injected in mice. We found that DHA is rapidly taken up by liver, selectively acylated into triglycerides and released back into the circulation in VLDL. Most of the DHA from VLDL and LDL appeared to be rapidly taken up by extrahepatic organs. This pattern seems to be unique for DHA, because no significant amount of non-essential oleic acid, traced in a similar way, was found in TG and VLDL fractions. In summary, these results point to the important role of VLDL and LDL in transport of DHA to extrahepatic tissues, and to the involvement of liver in the initial selectivity for DHA transport. A novel application of PET imaging for the study of C11-DHA incorporation into brain has been initiated. Brain and heart images from 19 healthy volunteers and 17 alcoholics have now been obtained. Extensive characterization of the fatty acid input function in plasma has been made in real time for the 11-C-DHA. Our findings thus far are that the J(in) and K* values for male and female healthy volunteers are similar except for the K* values in the thalamus and the gray matter/white matter ratio. Although there was a suggestion from initial studies that alcoholics may have a lower incorporation of DHA in many areas of cortex than control subjects, sufficent subjects could not be enrolled in the study to make this determeination due to funding constraints.